The present invention relates to a biaxially-oriented film, suitable for use as an electrically insulating film, composed of a highly-pure electrical film raw material based on polyolefins, in particular propylene polymers, which film contains a neutralizing agent. The invention also relates to a raw material which is suitable for producing this film, and to the use of the film.
The use of plastic films as electrically insulating materials has been known for a long time. These films are used as a dielectric in capacitors, for cable insulations or as self-adhesive insulating tapes. The requirements to be met by the electrical properties of such films are very stringent. They should have the lowest possible dielectric loss factor and the highest possible breakdown strength, it being required that these characteristic data be constant over time, especially at elevated temperatures, such as occur during the use of these films as intended. If the films, after application of a metal layer, are used in MKP-capacitors, good adhesion between the film surface and the metal is necessary.
Hitherto, attempts have been made to obtain the desired properties by the use of highly-pure raw materials, which have a very low residual ash content of less than 100 ppm, if possible, and are largely free of lubricants and ionogenic and polar constituents. The use of raw materials based on propylene polymers of this high purity for the manufacture of biaxially-oriented electrically insulating films has been disclosed, for example, in European Patent Applications 0,011,796 and 0,222,296.
However, even highly-pure polyolefin raw materials still have a residual content of catalyst residues, which are inevitably caused by the production process. Such catalyst constituents are especially compounds based on aluminum, titanium, magnesium and chlorine. When the raw material is used for the production of electrically insulating films, the active chlorine compounds present or being formed in the raw material must be neutralized, which is done in practice by the addition of calcium stearate.
However, the use of calcium stearate as a neutralizing agent leads to a marked increase in the dielectric loss factor of the electrically insulating film, as compared with the unneutralized material, especially if the film is used at elevated temperatures. Furthermore, the adhesive action, produced by a corona treatment, of the film surface towards metals such as aluminum or zinc with which capacitor films are coated, decreases in an undesired manner. The reason for this presumably is the migration of the materials converted in the neutralization reaction to the film surface in the course of time. The consequence is reduced adhesion of the metal layer and detachment of the metal. A further disadvantage is the evaporation of the stearic acid which is formed from calcium stearate in the neutralization reaction causing undesired depositions on machine parts during film production and film processing. These deposits can fall in the form of waxy drops onto the electrically insulating film and thereby cause a marked deterioration in the quality of the latter.